Why does Sirius sparkle with different colours?

[Ben478]

OK I know, beginner's question time now but I'm not afraid of asking!

I've observed Sirius on two occasions now and notice how it shines with a pure, brilliant silvery white colour, but also how it sparkles with other colours such as red and blue. Is this the star itself or as I suspect, the effect of the mirror and/or glass of the telescope or eyepiece? Many thanks, Ben.

[LukeSkywatcher]

Sirius twinkles because it is so low down to the horizon as seen from the UK. The light from it has to travel through a lot more atsmosphere which is full of dust and dirt. This is what gives it its colours. On nights when you see it as white, i guess this is a good indication of good "seeing" conditions.

[Starman]

Although the dust and dirt does change the colour of things, making them typically appear redder, the cause of the twinkling and colour flashing is due to the light from the star passing through layers and blobs of air with different light bending properties. As the light passes through this air it can be shifted by a small amount 'off-axis' if you like, and this causes a momentary change in the amount of light reaching your eye. The star appears to twinkle.

The bending can also be accompanied by the light being spread out into its component spectral colours and as this spectrum can also shift to the side, your eye picks the effect up as a flash of colour.

In theory the lack of twinkling (technical name for it is scintillation) could be seen as a sign of good seeing and I've had great views of the Moon and planets after using just that assumption. However, I've also had rubbish views when the stars have appears very steady in the sky so it's not a hard and fast rule at all.

Dust and dirt in the atmosphere isn't directly related to seeing but rather the clarity of the atmosphere, as described by the sky's "transparency".

[Starman]

I should also point out that the reason why Sirius appears to flash so much is that it's very bright and low. Being low, the amount of atmosphere it has to travel through is much thicker and hence there's more opportunity for the light from the star to get interfered with.

[Ben478]

Thank you both for your replies, it makes perfect sense now! So even though it's atmospherics to blame I think it makes Sirius even more beautiful to observe. It's just like a diamond when it sparkles!

[LukeSkywatcher]

Thanks for correcting me Pete. I'm always confusing "seeing" with "transparency".

[Starman]

No problem - an easy thing to do.

[LukeSkywatcher]

No problem - an easy thing to do.

You are too kind. I fear it is just me though. I have a terrible habit also of confusing Nagler EPs with Kelner. I usually end up saying something like "The nagler EPs that come supplied with most scopes are pretty bad".

[ollypenrice]

Splendid answer from Pete. It would be interesting to see Sirius from much further south so as to see iit near the zenith. An non-scinillating Sirius would be most unfamiliar.

Olly

[themos]

Our eyes can't detect colour if the source is dimmer than some threshold. We don't see the scattered colours from other stars because they're not as bright as Sirius even when they are near the horizon.

[marcdearn]

I saw Sirius (and the rest of Orion) in Goa...it took a while to recognise it as it was so high

[Towa]

For a real challenge you could have a go at seeing the extremely

difficult white dwarf companion that orbits Sirius.

Here's a drawing I made last year, the Pup was visible in the glare of the Sirius.

[Talitha]

It's beautiful when they flash like that isn't it, Ben? I've also seen it happen with Capella, Rigel, Vega, Spica and Venus. At least our atmospheric gunk is worth something.

[Ben478]

Ah Rigel, that's another on my list - blue/white isn't it?

[ollypenrice]

Well done to Towa. Fabulous observation, that.

Olly

[Eriska]

Hi! Im viewing sirius from Vancouver Island, Canada. 49th parallel. It is flashing colours like crazy! It is definitely not low in the sky right now. Could it be because it's much colder than usual tonight? Maybe it really does flash different colours? So beautiful.

[ollypenrice]

It's called scintillation and has nothing to do with Sirius itself, which is a stable star. The atmosphere is turbulent and causes diffraction which will affect the different wavelengths of light differently, causing the beam from Sirius to move around, dim and brighten, and change colour. The lower a star is in the sky the more atmosphere its beam has to pass through, so scintillation decreases as we look upwards towards the zenith. The stability/instability of the atmosphere at a particular time is known as the 'seeing' - which is highly variable. When it's bad there is no point in using high magnification. 

Planets don't twinkle because they are discs of light rather than points, like stars, so they send out many parallel beams each of which are affected randomly by the atmosphere so that the scintillation averages itself out.

Sirius itself does not change colour or brightness rapidly. This is quite complicated but there are no physical processes in stars which can bring about changes on that timescale. 

Olly

[spaceman_spiff]

There's a great article on the subject:

http://www.skyandtelescope.com/astronomy-blogs/sirius-ly-scintillating-holiday12222014/

@Eriska- here's a photo from the article showing the apparent colours of Sirius.

[Whistlin Bob]

On 25/01/2010 at 21:25, Towa said:

For a real challenge you could have a go at seeing the extremely

difficult white dwarf companion that orbits Sirius

Kudos indeed! I tried this for some time a few weeks back but no joy. Also tried mucking about with camera settings to see if I could image it instead but again didn't manage it. One of those fails where you don't really mind 'coz you had fun trying.

[Olli]

Wanted to know this as well thanks for bringing it up can always see it with red/blue sparkle with the naked eye but never knew why.  

[timwetherell]

Yes Sirius B is a very challenging target in the UK with Sirius so low in the sky! I finally managed it visually a couple of nights ago with my 7" refractor but I've tried many times before to no avail. It all comes down to seeing. In moments of very steady air it's right there and relatively obvious but most of the time it's lost in a sea of scattered and dispersed light from Sirius. For some reason the atmospheric dispersion and turbulence tend to smear the light into a flame or fan shape like a peacock tail, which would be very pretty if it's wasn't masking what you were looking at!

[AlentejoSkies]

Down here in the southern Alentejo, Portugal at more or less 37 degrees compared to southern UK 51 degrees North, Sirius is way up high right now, and sparkling like crazy anyway. I love the interaction with the atmosphere, with or without a telescope, but most of all through a telescope way off focus, like a plasma ball, so beautiful!

[trynda1701]

On 19/02/2018 at 13:21, spaceman_spiff said:

There's a great article on the subject:

http://www.skyandtelescope.com/astronomy-blogs/sirius-ly-scintillating-holiday12222014/

@Eriska- here's a photo from the article showing the apparent colours of Sirius.

That is a cool set of images, shows how much scintillation can change the colour of the light!

As an aside, that middle image looks a lot like Mars! And now since I've thought that, all the others now look like the old versions of the planets you saw on the original series of Star Trek before they updated the effects!

[kirkster501]

Sirius also appears to sparkle much more because it is so bright and is a point source of light.  Of course, stars with a similar declination will also twinkle but less so than Sirius.

The planets do not twinkle nearly as much (though do a bit) because they are disks.

[Mamabere]

Hey love that you posted this as you are not far from me,  (cowichan valley) and I see and enjoy serius sparkling all the time.  I've often wondered why it did.  Thank you all for sharing your knowledge! ?